Method of separating ammonia from gases and mixtures of gases containing ammonia



Feb. 4, 1930. E 1,745,730

METHOD OF SEPARATING AMMONIA FROM GASES AND MIXTURES 0F GASES CONTAINING AMMONIA Filed June 13, 192"! pir -2+H2 'Renchon chamber HIH3+ 2 'N2+Hz 5 'Fre-cooler Wafer c fled exchanger Absorber r Nz+Hz WG+ZVM L Xchanger\ I J, J, MHz 05 NH3 NzPHZ \f v A Vacuum pum EXPUPLSIOH valve //wen fan Qzymz Patented Feb. 4, 1930 GEORG FRIEDRICH UHDE, OF BOVINGHAUSEN, GERMANY METHOD OF SEPARATING AMMONIA FRO! AND MIXTURES CONTADIING AHIONIA OF GASES Application filed. June 13, 1927, serial No. 198,645, and in Germany June 15, 1926.

, In executing the synthesis of ammonia from nitrogen and hydrogen under pressure, the ammonia obtained is usually eliminated from the current'of gas by washmg the gas, while under pressure, with water. This mode of execution is onl feasible in cases where catalysts are emplbyed in the synthesis, upon which the water vapor contained in the washed gas does not exercise any detrimen- 0 taI effect, because the washed gas, that is, uncombined nitrogen and hydrogen, must again be conducted into the catalyzing process.

Hitherto it was usual to separate the ammonia from the gas under pressure by cooling down the latter to the temperature of the surrounding atmosphere or somewhat there- 7 below and conducting the remaining gas back again into the catalyzing process. In view of the high percentage of ammonia still contained in the remaining as returned to the process, the formation 0 further quantities of ammonia in passing the gases over the,

catalyst was no favorable. Assuming for instance that the equilibrium has been reached, only the difference between the quantity. of ammonia, corresponding to the equilibrium and the quantity of ammonia,-already present in the reactivegas. mixture can be formed.

Actually, it has been shown by Larson and Black (Journal of the American Chemical Society-1925) that the vapor tension of ammonia at elevated pressures, in the neighborhood of 100 atmospheres, is very high, as

much as six times the vapor tension under normal conditions. This means that in an ammonia-nitrogen-hydrogen mixture at normal temperature and under a hundred at- 'mospheres pressure, the concentration of 40 gaseous ammonia is as much as 3 to 4 percent by volnme. Therefore, return of the gaseous mixture to the catalyst does not result in an equilibrium conversion of the available nitrogen and hydrogen because 30 to 4 of the ammonia in the catalyzed gas un- .invention, the evaporation-heat of the ammonia separated in the liquid state is utilized for the purpose of cooling down and con- (lensing the gases. This is effected in such a manner, that first the incompletely separated liquid ammonia from the precooled c5 mixture of gas is eliminated from the mixture of gas for instance in a suitable vessel, thereafter the ammonia is expanded to normal pressure and fed to meet the arriving reactionsgas mixture for the exchange of heat in a heat exchanger. In this manner the arriving gases, previously cooled down by water to normal temperature, are further cooled to minus 33 degrees C. The content of the ammonia remaining in the gases is correspondingly lowered. If desired a: greater quantity of ammonia may intentionally be added to the gas mixture which is to be cooled.

Of course the low temperature of the gas freed from ammonia may be utilized to precool the arriving gas in a heat exchangesystem.

By the present invention a further important progress is thereby obtained, in that the above mentioned separatedliquid ammonia may; be expanded into an evacuated heat exchange-system. In this manner the cooling action and therefore also the separation of the ammonia from the gas mixture under pressure may be greatly improved. In this 90 such an extent that but 0.1 to 0.3

said valve i manner it is possible to obtain a temperature .of minus 70 C. in the gas outflowing from the catalysts, i. e. nearly to the freezing point of the ammonia from the gas-mixture to er cent of ammonia by volume remains in t e gases returned to the catalyst.

During the reassage of the gas-mixture of nitrogen and by rogen so far freed from ammonia that it contains only 0.1 to 0.3 percentv of ammonia by volume over the catalyst a considerably higher efliciency is obtained. Assuming that nearly 10 volume-percent of ammonia are produced after a single passage of st, 97 to 99% p of t equilibrium quantity 0 ammonia are produced during the re-pasfie of the gases over the catalyst. On the 0 er hand only -70% of ammonia could be newly progo duced in passing the gas mixture over the catalyst according to the old method, in which the remaining gas mixture still contains 3 to 4 volume-percent. It could not be expected that it was possible to succeed in this simple as and before all so less expensive way to free the gas mixture going out from the catalyst so far from ammonia.

For a better understanding the single steps of the method are more clearly illustrated in the annexed drawin diagrammatically showing a'plant adapt for the execution of the new method.

First the gas mixture under pressure containing ammonia is cooled by a preliminary cooling with water. For this purpose the gas leaving the catalyst-furnace a and contaming ammonia is cooled in a cooling vessel b by means of water, surrounding the coil provided in the vessel b, so that a artial quantity of ammonia is separated. e gas still charged with ammonia is then led through the heatexchanger c and d assing. through the pipecoils of and d. T e se arated ammonia is collected in the vessel e. 18 exhausted at the bottom of t e vessel e by means of a valve 0 and expanded behind the a ipe-conduit into which the coil (1 is inse and which leads to the vacuumpump 9, so that in this branch of the pipe conduit gaseous ammonia is under vacuum. This, cold ammonia is led in counter-current heat exchange relationship with the gas arrivrng through the coil 11; in this manner the arriving gas and the ammonia se rated therefrom are constantly cooled, so that finally the liquid ammonia collected in the vessel 0 has a tem rature from minus 60 to centlgrades. is temperature automatically results by vaporizing the ammonia in the vacuum as above described. In this manner a m xture of gas having a temperature from minus 60 to 70 centigrades accumulates in the vessel 0, likewise transmittin its cold to the gas arriving through the coi c in the exchanger c traversing the latter through the his li uid ammonia coil 0 thereby increasing the cooling action. The reactive gas leaving the vesselc at its top and consisting of nitrogen and hydrogen is thereby rendered nearly free from ammonia, because the gas-mixture contains at atemperature of minus 7 0 centigrades in average only 0.1 to- 0.15 volume percent of ammonia. The

,ifrom mixtures of the formative gases, the

method of separating ammonia from the gaseous reaction. products which comprises, precooling the gases while under pressure to ordinary templerature, passing the pre-cooled gases into eat exchange relationshi with cold nitrogen and hydrogen separat previously cooled quantities of the reaction gases, the heat exchange bein carried out substantially in the absence 0 any heat exchaii e relationship with ammonia, then passing t e reaction mixture into heat exch relationship with cold gaseous ammonia o tained from the vaporization under low pressure of li uid ammonia se arated from previousl coo ed quantities o the reaction gases, the eat exchange with ammonia being carried out substantially in the absence of any heat exchange relationshi with the nitrogenhydrogen mixture, where the ammonia in the reaction gases is lique ed, collecting the liquefied ammonia, returning the nitrogen and hydrogen separated therefrom into heat exchange relationship with further quantities of pre-cooled gases, vaporizing the liquid ammonia under lowpressure, and conducting the cold vaporized ammonia having a temrature between minus 60 and'70 G. into eat exchange relationship withv further quantities ofreaction gases to be cooled.

2. In the process of synthesizing ammonia from mixtures of the formative gases, the method -of separating the produced ammonia from the gaseous reaction mixture, which comprises 'n the reaction mixture into heat exchange reationship with cold nitrogen-hyd gen mixture se arated from previously cooled quantities 0? reaction the heat exchange being-carried outsubstantially in the absence of any heat exchan e relationshi of the reaction mixture wit ammonia an then 'passin heat exchange re ationshifi with cold gaseous ammonia obtained from t e vaporization under low pressureof liquid ammonia, at a temperature close to ireezing point of ammonia which is ac arated from previously cooled quantities o the reaction mixture, the heat exch with ammonia heir? carried out substantially in the absence 0 any heat exthe reaction mixture into.

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